As common as they are, the origins of allergies are poorly understood. The prevailing theory is that they evolved as a method of defense against macroparasites — like ticks and worms — that are now triggered by a much wider variety of causes. However, a group of researchers from Yale think we're looking at it wrong, and allergies are actually a multifaceted defense system, designed to protect us from all sorts of horrible biohazards.

They argue that while stopping big parasites was doubtless part of it, that doesn't do much to explain our reaction to things like nuts, which have no obvious connection to pests. It also doesn't give a hint as to why we need something as rapid as anaphylaxis to fight of slow moving, and slow replicating parasites.

Rather than just parasites big enough to see with the human eye, this group believes allergies are a defense against helminthes; noxious xenobiotics; venoms and haematophagous fluids; and environmental irritants.

The allergic reactions are then ways of dealing with these problems. Mucus secretion enhances your bodies barriers; sneezing, coughing, and vomiting can expel the irritant; white blood cells can directly kill invaders; and your body can change to restrict spread and repair tissue. Since this whole process is remarkably unpleasant, it also works to teach you to avoid the trigger. But what about anaphylaxis? A sudden, system wide shutdown is arguably a defense against a deadly venom to limit its damage.

While it's not a complete explanation for widespread rise in allergies, nor why we react so heavily to seemingly minor triggers, but it does provide an explanation why so many things seem to set it off.